1. Field of the Invention
The present invention relates to an axial flow turbine used as a steam turbine, a gas turbine or the like for a power-generating plant.
2. Description of the Related Art
An improvement in the power generation efficiency of the power-generating plant has recently led to a strong demand for further improved turbine performance. The turbine performance has a relationship with a stage loss, an exhaust loss, a mechanical loss and the like associated with the turbine, and it is considered most effective to reduce the stage loss among them for further improvement.
The stage loss includes various losses, which are broadly divided into:
(1) a profile loss attributable to airfoil per se,
(2) a secondary flow loss attributable to a flow not along with a main flow, and
(3) a leakage loss caused by working fluid (steam, gas or the like) leaking to outside the main passage.
The above leakage loss includes:
(a) a bypass loss caused by a portion (leaking fluid) of the working fluid flowing through a clearance passage (a bypass passage) other than the main passage, making the energy in the leaking fluid not effectively utilized.
(b) a mixing loss caused when the leaking fluid flows from the clearance passage into the main passage; and
(c) an interference loss caused by the interference of the leaking fluid flowing into the main passage with a blade row on the downstream side thereof.
An important issue in recent years is to reduce not only the bypass loss but the mixing loss and the interference loss. In other words, the important issue is not only to simply reduce the flow rate (a leakage amount) of the leaking fluid from the main passage into the clearance passage but how to return the leaking fluid from the clearance passage into the main passage with no loss.
To solve such problems, it is proposed that a plurality of guide plates is provided on the downstream side of the clearance passage so as to change the flowing direction of the leaking fluid to the main flow direction. (See JP-2011-106474-A)